Chocolate Cherry Cobbler Sundae

1 Tbsp. cornstarch
1 - 1/4 cups sugar, divided
6 cups pitted fresh cherries
1/2 cup butter, softened
2 large eggs
1 tsp. vanilla extract
1 cups all-purpose flour
1/4 cup unsweetened cocoa
1/4 tsp. salt
1/2 cup buttermilk
1 - 1/2 tsp. white vinegar
1/2 tsp. baking soda

Preheat oven to 350 degrees. Stir together cornstarch and 1/2 cup sugar. Toss cherries with cornstarch mixture and spoon into lightly greased 11x17 inch baking dish. Beat butter at medium speed until fluffy; gradually add remaining 3/4 cup sugar, beating well. Add eggs, one at a time, beating just until blended after each addition. Stir in vanilla until blended. Combine flour, cocoa, and salt. Stir together buttermilk, vinegar and baking soda in a separate bowl. Add flour mixture to butter mixture, alternating with buttermilk mixture, beginning and ending with flour mixture. Beat at a low speed until blended after each addition. Spoon batter over cherry mixture. Bake for 45 to 50 minutes. Cool for 10 minutes. Place into a sundae dish. Top with vanilla ice cream and hot fudge sauce.